The invention relates to adaptive modulation and coding (AMC), and in particular, to a cross layer AMC method combining media access control (MAC) and physical (PHY) layers to enhance system performance.
AMC provides the flexibility to match the modulation-coding scheme to the average channel conditions for each user. With AMC, the power of the transmitted signal is held constant over a frame interval, and the modulation and coding rate is changed to match the current received signal quality or channel conditions. Different order modulations allow transition of more bits per symbol, thus, higher throughputs and better spectral efficiencies are provided. When using a modulation technique such as QAM64, however, better signal-to-noise ratios (SNRs) are needed to overcome any interference and maintain a certain bit error ratio (BER). The use of adaptive modulation allows a wireless system to choose the highest order modulation depending on the channel conditions, as range increases, to step down to lower modulations. Conversely, a closer target can utilize higher order modulations such as QAM for increased throughput. Additionally, adaptive modulation allows the system to overcome fading and other interference.
FIG. 1 shows the environment for adaptive modulation and coding (AMC) scheme. In a system with AMC, users close to the transmitter 100 are typically assigned higher order modulation with higher coding rates (e.g. QAM64 with R=3/4 turbo codes), but the modulation-order and/or coding rate will decrease as the distance from the transmitter increases, such as QAM16, QPSK and BPSK.
The implementation of AMC offers several challenges. AMC is sensitive to measurement error and delay. In order to select the appropriate modulation, the scheduler must be aware of the channel quality. Errors in the channel estimate will cause the scheduler to select the wrong data rate or transmit at too high a power, wasting system capacity, or too low a power, raising the block error rate. Delay in reporting channel measurements also reduces the reliability of the channel quality estimate due to the constantly varying mobile channel. Changes in the interference also add to the measurement errors.